Starch-based degradable plastic film

Starch from cereal grains such as corn and wheat shows promise as a component of plastics. especially where the introduction of a biodegradable component is important. For many years up to the early 70s, petroleum was readily available at $2 to $4 per barrel and starch sold for about 5 cents per pound. Since that time. petroleum has increased 5- to 10-fold while starch maintained a steady price or increased little more than 2-fold. These price changes and increased availability now make starch economically attractive as a substitute for petroleum.

Most common starches contain 17-27% amylose. a linear polysaccharide consisting of 400-1.000 1A-linked a-D-glucose units, and 73-82% amylopectin. a branched molecule consisting of D-glucose units linked a-D( 1-4) with branches at the C-6 position once every 26 or more D-glucose units. Starch readily gelatinizes (or disperses) in hot water to form a paste that can be cast into film. However, such films are sensitive to water and become quite brittle upon drying due to retrogradation of the starch. To alter these properties. starch has been blended with water-soluble plasticizers and synthetic polymers.

Unfortunately. starch is quite incompatible with most commercially available synthetic resins used in plastic production such as low density polyethylene (LDPE). Aqueous blends of gelatinized starch, polyvinyl alcohol and glycerol or other polyols can be cast into flexible films (1) . For large-scale applications. water-extractable plasticizers must be avoided. and the for-. mulations must be adaptable to more economically feasible extrusion blowmg techniques now used for most synthetic film production. Also. relatively high levels of starch (40% or more) may be required to achieve desirable rates of biodegradation. We have had considerable success in achieving these objectives by blending gelatinized starch with poly(ethylene-co-acrylic acid) (EAA).

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Films made from this system require no plasticizer. yet they remain flexible even after exposure to water and drying. Although the mechanism is not known'. we envision that as internal bonding within the starch molecule is reduced during gelatinization and extrusion. new bonds are formed between the starch hydroxyls and the EAA carboxyl groups that retard retrogradation of the starch.

Starch not only serves as a biodegradable. renewable component but reduces oxygen perme;bility and can introduce semipermeable characteristics into the film. Starch-containing films may have application as agricultural mulch for which the removal and disposal of conventional films is expensive and disposal sites are increasingly difficult to find.

Film Preparation. Several techniques have been explored for making the films. Initially. they were made by casting aqueous dispersions of starch and EAA (2). These films are promising for applications where up to 90% starch is needed for rapid biodegradation but where strength. flexibility. and water resistance are not critical. When lower levels of starch were used, good physical properties and water resistance were achieved; however. the cost could prohibit certain large-scale applications because casting is a relatively slow process and aqueous dispersions of EAA are more expensive than pelletized EAA.

A second method. which allowed the use of EAA pellets, involved premixing film formulations with excess water (50% or less solids concentration) at 9O-IOO°C in heavy-duty Sigmaor Banbury-style mixers for 45 minutes (3). The resulting doughlike product was repeatedly extrusion-processed into strands until the moisture content of the extruded product was about 5 to 10%. The product was then blown into film by extruding it through a 0.5-inch blown film die. Small amounts of alkali were essential to obtaining quality film. Ammonium hydroxide is the preferred alkali for general purpose film because any excess readily escapes during processing of the films. When strong alkali is added. such as sodium hydroxide. the resulting films have semipermeable properties. This premixing method allows the addition of various formulating aids such as LDPE to lower the formulation cost or various polyols and sugars to increase the amounts of biodegradable composition. Films have been made by this method with up to 60% starch: however, the preferred maximum starch level is about 40-50%.

One major concern to potential manufacturers was the need to premix the starch and EAA in a heavy-duty mixer with large amounts of water before extrusion processing and blowing into film. Quality films have now been made without this premixing step and with much lower initial moisture. by incorporating urea into the system. The major function of the urea is to reduce the amount of water and time needed to gelatinize the starch. Using this technique. a 35-1b. pilot-plant run was successfully completed. A semidry composition of 45% EAA. 40% starch. and 15% urea plus about 17 parts per hundred of aqueous ammonium hydroxide was mixed in a ribbon blender and then extruded into small strands. The extruded product was fed through a commercial blown-film extruder by Rex Plastics, Thomasville, N.C. The extruder had a 2-inch diameter screw and a 4-inch die. The bubble had a lay-flat width of 12 inches.

Film Properties. In general, starch-EAA films had tensile strengths that remained constant at 2000-2300 psi. but their percentage elongation dropped from 130% to 20% as the starch level was increased from 20% to 50%. Substituting up to half of the EAA with LDPE caused some loss in both strength and percentage elongation, especially with aging; however, cost reduction may outweigh these slight property losses.

Incorporating 10-15% urea into a starch-EAA formulation. to eliminate the heavy-duty mixing step, lowered tensile strengths of films with 40% starch to about 1600 psi, but the percentage elongation increased slightly due to the plasticizing effect of urea. After soaking in water, which removed the urea, tensile strengths and percentage elongations were comparable to those made, without urea, by the premixing method.

No field studies have been conducted on the films to determine effect of long-term outdoor exposure on decomposition rates. In laboratory studies, films containing 40% or more starch develop mold growth within one month after being sprayed with a mixture of microorganisms. and they lost 50% or more of their mechanical properties when in contact with soil for a few months.

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10.05.2010. 23:50

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